This invention relates to fireplace structures in general and in particular either a space heater type or direct radiation masonry fireplace or one in which a fireplace heater unit develops heated air to be re-circulated within a room. Various types of fireplace units of this latter type are known and consist essentially of a firebox with surrounding duct work as shown, for example, in the Libbey U.S. Pat. No. 438,319. In fireplaces of this type or of the space heater type, as exemplified by Derringer et al. U.S. Pat. No. 3,654,913, a considerable amount of room air is used to support combustion within the combustion chamber. This air is supplied by the room in which the unit is located and any adjacent rooms can create drafts within the room since the natural convection of a burning fire draws air into the firebox and thence up the flue. Drafts are uncomfortable for one drawing benefit from the heat generator such as a space heater or fireplace, and the only prior recognition of this problem is in the Hoffman U.S. Pat. No. 2,863,443. The Hoffman patent which is concerned with a stove insert into a masonry structure provides a direct opening underneath the firebox where ashes can directly fall and clog the inlet. It is not completely satisfactory due to the location of the air inlet source.